Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle in eukaryotes, which are activated by binding to cyclins whose abundance changes during the cell cycle. A single cyclin and multiple CDKs function in yeast cell cycle control, whereas multiple cyclins and multiple CDKs control the cell cycle progression in animals. Trypanosomes appear to employ a cell cycle control system different from fungi and animals. A PHO80-like cyclin, CYC2, and a CDK-related kinase, CRK1, control the G1/S transition, whereas a B-type cyclin, CYC6, and another CDK-related kinase, CRK3, govern the G2/M transition. However, how the two CRKs exert their roles in cell cycle progression remains elusive, mainly because their downstream targets are not known. The current proposal aims to understand the regulation of G1/S and G2/M transitions by CRK1 and CRK3. In aim 1 of this proposal, we plan to establish the chemical genetic approach in trypanosomes by creating ATP analog sensitive mutants of CRK1 and CRK3. Aim 2 is to dissect the roles of CRK1 in the G1/S transition by investigating CRK1-mediated regulation of the Cdc45/Mcm2-7/GINS complex for DNA replication initiation. Aim 3 is to examine the roles of CRK3 in the G2/M transition by characterizing CRK3-mediated regulation of the chromosomal passenger complex and its implication in mitotic entry. Through molecular, cell biological, chemical genetic, and biochemical approaches, our overall goal in this proposal is to understand the mechanistic roles of CRK1 in the G1/S control through regulating the DNA replicative helicase, the Cdc45/Mcm2-7/GINS complex, and CRK3 in the G2/M control through regulating the chromosomal passenger complex. The outcomes from these studies not only would significantly further our understanding of the mechanisms of cell cycle transitions in trypanosomes, but also could validate CRK1 and CRK3 as drug targets for anti-trypanosome chemotherapy.